The present invention relates to a position detecting apparatus used for detecting a rotational position of a machine shaft.
FIG. 1 is a block diagram showing an embodiment of a prior art position detecting apparatus, and FIG. 2 is a time chart for a timing controller 1, which is an essential component of the apparatus of FIG. 1. An exciting signal VC is generated by an exciting generator 10 upon receipt of an exciting generation signal VG output by the timing controller 1 at a time point t.sub.1, amplified by an amplifier 2 to become an exciting signal v for a resolver 3 and fed to the resolver 3. Analog signals v'. sin .theta. and v'. cos .theta. which serve as functions for an angle of deviation .theta. from a reference position are respectively output to amplifiers 5S and 5C from the resolver 3 by the exciting signal v and are amplified. The amplified analog signals v'. sin .theta. and v'. cos .theta. are respectively held by sample-and-hold circuits 6S and 6C in accordance with a hold signal HD output by the timing controller 1 at a time point t.sub.2. The held analog signals v'. sin .theta. and v'. cos .theta. are respectively output to A/D converters 7S and 7C and are converted from analog to digital signals in accordance with an A/D signal ADS output by the timing controller 1 at a time point t.sub.3. The converted digital signals A and B are respectively latched in memories 8S and 8C in accordance with a data latch signal DLT output by the timing controller 1 at a time point t.sub.4. An arithmetic unit 9 calculates tan.sup.-1 (A/B) to output the deviation angle .theta..
Since errors are generally included in the deviation angle .theta. due to off-set values which are generated in the amplifiers 5S and 5C, the sample-and-hold circuits 6S and 6C and the A/D converters 7S and 7C, variable resistors 4S and 4C are provided in the amplifiers 5S and 5C and are ajusted so as to cancel the off-set values.
The above prior art position detecting apparatus cannot cope with changes in off-set values caused by changes in temperature and fails to detect the position with a high precision. Further, adjustment by variable resistors is too troublesome.